python Lookup instance attribute

gistfile1.txt

In [88]: class C:
   ....:     def foo(self):
   ....:         pass
   ....:     foo = classmethod(foo)
   ....:

In [89]: C.__dict__
Out[89]:
{'__doc__': None,
 '__module__': '__main__',
 'foo': <classmethod at 0x102fbd868>}

In [90]: C.foo
Out[90]: <bound method classobj.foo of <class __main__.C at 0x102fd0050>>

In [91]: class C:
   ....:     def foo(self):
   ....:         pass
   ....:

In [94]: C.__dict__
Out[94]: {'__doc__': None, '__module__': '__main__', 'foo': <function __main__.foo>}

In [95]: C.foo
Out[95]: <unbound method C.foo>

non-data descriptor能够被同名attribute覆盖 而data descriptor不能

If attrname is a special (i.e. Python-provided) attribute for objectname, return it.

Check objectname.__class__.__dict__ for attrname. If it exists and is a data-descriptor, return the descriptor result. Search all bases of objectname.__class__ for the same case.

Check objectname.__dict__ for attrname, and return if found. If objectname is a class, search its bases too. If it is a class and a descriptor exists in it or its bases, return the descriptor result.

Check objectname.__class__.__dict__ for attrname. If it exists and is a non-data descriptor, return the descriptor result. If it exists, and is not a descriptor, just return it. If it exists and is a data descriptor, we shouldn't be here because we would have returned at point 2. Search all bases of objectname.__class__ for same case.

Raise AttributeError
http://www.cafepy.com/article/python_attributes_and_methods/python_attributes_and_methods.html
http://python-history.blogspot.jp/2010/06/inside-story-on-new-style-classes.html